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Can you tell me if this amp design will work

  1. Nov 17, 2016 #1
    I am struggling with this a bit. I think this will work but I am not sure. Can you guys take a look and see if I have made any obvious mistakes. The B+ voltage should be around 480 VDC loaded. I am still working on the channel switching.





    Last edited by a moderator: May 8, 2017
  2. jcsd
  3. Nov 17, 2016 #2
    Sorry . I posted the preamp section twice and don't know how to remove it. Here is the PA.

  4. Nov 17, 2016 #3


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    Output transformer secondary 'T' is floating, it has only one feedback connection, is the other terminal grounded?

    No ceramic bypass caps on the LM7805 regulator. It will howl at the moon.

    The LM7805 has no reservoir capacitor.
    The LM7805 needs a minimum input of 7.5VDC but it has only 6.3VAC.
    6.3VAC * Sqrt(2) = 8.9 volts peak, less two diode drops of 1.1V each, in bridge = 6.7 volts
    You have run out of voltage headroom. Regulator will always be in drop-out.
    Headroom decides value of reservoir capacitor. C = I * dt / dVripple

    Edit: Or is that secondary 6.3-0-6.3 giving 12.6VAC without showing the CT ?
    Last edited: Nov 17, 2016
  5. Nov 17, 2016 #4
    Thanks for the feedback. Yes the other terminal is ground, just not shown. I will go back and take another look at the LM7805 issue. I may have to adjust several things because I bit the bullet and ordered high end Mercury Magnetics transformers. $658.00 plus the shipping from Cali to Florida. MM is pretty pricy. I was shooting for around 480 to 500 VDC loaded.
    All the parts should arrive next week by Tuesday.
    I ordered a blank chassis so I got some machining to do also.
    I am trying to have this up and running and debugged by the 28th. Lots of work ahead.


  6. Nov 18, 2016 #5


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    The ISOLATE GRD is connected to Ground thru R72, R73, and the rectifier bridge.

    With the values shown, the filter caps C34, C35 will result in 1.5Vpp ripple to the preamp heaters.
    Last edited: Nov 18, 2016
  7. Nov 18, 2016 #6


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    The whole complex around V6 and the feedback loop makes me go "unstable!" Exactly where (physically) are you connecting the output transformer secondary to ground? That detail could be the difference between "motorboating" and silent when no signal is present.

    Again, I would never design an amplifier without current feedback in the output tubes (at least do a DC feedback to stabilize the thing). I am also extremely wary of designing a feedback from the transformer secondary due to the phase shift in the transformer. My favorite feedback in the output stage is to the g2. Take a look at http://www.dogstar.dantimax.dk/tubestuf/driverpa.htm
  8. Nov 18, 2016 #7
    Thanks so much guys. I am at the Dr. with my wife ATM and will respond later this evening.
    Life is in full session today...lol I will never get use to typing on a cell phone...lol
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